Autoimmune diseases can develop upon defects in the process of tolerance, a process that eliminates and control self-reactive lymphocytes. Novel experimental therapies that aim at depleting B cells with specific antibodies have emphasized the important role that B lymphocytes play in the pathogenesis of autoimmunity. However, most of our knowledge of the process of B cell tolerance and B cell selection in humans originates from studies of the mouse immune system and, in particular, of the immune system of immunoglobulin transgenic mice, because in these systems we can follow the development of self-antigen-specific B cells in the presence or absence of self-antigen. Which mechanisms control autoreactive human B cells has implication on the etiology of autoimmune diseases and, so far, most studies of autoimmune patients were unable to reconstruct the primary defects that have originally led to illness. These studies reveal gaps in the methodology available to investigators for the analysis of the immune tolerance process in healthy and diseased human individuals, and that it would be useful to have an in vivo system in which to characterize mechanistically how human B cells undergo tolerance. Importantly, such system would also allow us in the future to determine whether patients with autoimmune diseases manifest defects in specific tolerance mechanisms and to test possible therapies to prevent and/or correct the pathological manifestations of such defects. In the past, anti-immunoglobulin antibody treatment has been used in mice and other animals as a way to test whether B cells undergo tolerance upon antigen receptor ligation. Recently, transgenic mice expressing superantigens reactive to antigen receptor constant regions have been used to understand how tolerance is attained in normal B cells of polyclonal immune systems. Moreover, there has been quite impressive recent progress in the methodology to generate human/mouse chimeras (humanized mice) in which human hematopoietic stem cells reconstitute severe combined immunodeficient mice. This system holds tremendous promises for more detailed studies of the human immune system, including human B cells. Here we propose to characterize the feasibility of using humanized mice for the study of B cell development. Furthermore, we propose to combine the anti-immunoglobulin treatment and the superantigen transgenic technology with the human/mouse chimera methodology to generate a novel system to analyze human B cell central and peripheral tolerance in vivo. Once this novel system has been fully developed and characterized, we aim in future studies to determine the mechanisms of tolerance of human B cells from healthy and diseased individuals. PUBLIC HEALTH RELEVANCE Systemic autoimmune diseases such as lupus erythematosus and rheumatoid arthritis are complex disorders caused by defects in B and other blood cell types and that continue to cause significant morbidity and mortality. Although significant knowledge on these conditions has been achieved, substantial work remains to be done to fully understand the etiology and pathogenesis of these diseases and that will allow the development of novel and more targeted therapeutics. Our studies aim at developing and characterize novel research systems that may help us to understand at the mechanistic level the process that governs selection and regulation of B cells in healthy and diseased individuals. These novel systems will also be amenable for testing novel potential therapeutics on patient cells.